Arsenic is naturally present in petroleum feedstocks. Although the concentration of arsenic is low in most petroleum feeds, some crude oils, especially those found on the West Coast of the United States, contain high levels of arsenic. Some shale oils are also known to have high levels of arsenic. In these and other feedstocks, arsenic is usually found in the form of organo-metallic complexes and is present in light petroleum fractions such as naphtha and distillates. This arsenic is deposited on hydrotreating catalysts during operations designed to remove sulfur and nitrogen compounds.
The deposition of only 0.1 wt. % arsenic on a hydrotreating catalyst can cause significant catalyst deactivation. In some services, as little as 0.5 wt. % arsenic deposited on a catalyst can cause the loss of more than 70% of such catalyst' activity. Refiners cope with this problem by regularly replacing conventional hydrotreating catalysts that have been poisoned by arsenic. Heretofore, catalysts having specific activity and selectivity for arsenic removal have not been developed. The present invention provides for a catalyst that is effective at selectively removing arsenic from petroleum feedstocks, particularly from naphtha and light distillates, while simultaneously catalyzing the removal of sulfur and nitrogen.
Some researchers have taught hydrotreating catalysts that bear a superficial similarity to the catalyst of the present invention. For example, in U.S. Pat. No. 4,048,155, O'Hara teaches a desulfurization catalyst comprising an inorganic oxide carrier, a Group VIB metal component and a Group VIII metal component that is prepared by extruding at least 10% of the Group VIII metal component with the inorganic oxide carrier material and then finished by impregnating the resulting extrudate with the Group VIB metal and remaining Group VIII metal components.
More recently, U.S. Pat. Nos. 5,389,595 and 5,494,568 (Simpson, et al.) and U.S. Pat. No. 5,686,375 (Iyer, et al.) have taught a hydrotreating catalyst, hydrotreating process and method of making said catalyst wherein the catalyst comprises a porous refractory support containing an underbedded Group VIII metal-containing compound also impregnated with a Group VIB metal.